Cryogenic thermal switch

ABSTRACT

A thermal switch for linking a source of cryogenic refrigeration to a load. A copper plunger operating in a pressurized helium atmosphere provides a thermal link between a heavy-walled copper heat station cooled by a refrigeration source and one or more successive thick walled copper heat stations thermally separated by thin-walled stainless steel sections. Movement of the plunger can be used to control the temperature of the successive heat stations and the load associated with them. The thermal switch and its linear driving mechanism may be located within different physical environments and the switch itself is adapted to cross barriers defining different physical environments.

Hosmer et al.

CRYOGENIC THERMAL SWITCH Feb. 20, 1973 Primary Examiner-Harold Broome [75] Inventors: Thomas P. Hosmer, Concord; John F. Peterson, Lynnfield, both of Amey Bess'e Leppe' A Mass i T h I 57 ABSTRACT [73] ssignee' $33532: no A thermal switch for linking a source of cryogenic refrigeration to a load. A copper plunger operating in Flledi P 1971 a pressurized helium atmosphere provides a thermal [21] APPL N03 357 link between a heavy-walled copper heat station cooled by a refrigeration source and one or more successive thick walled copper heat stations thermally US. Cl- ..165/96, eparated steel sections [51] Int. Cl. ..F28f 27/00 Movement of the plunger can be used to controlpthe [58] Field 01 Search ..62/383; 165/32, 96 temperature f the successive heat Stations and the load associated with them. The thermal switch and its [56.] References Cited linear driving mechanism may be located within different physical environments and the switch, itself is UNITED STATES PATENTS adapted to cross barriers defining different'physical 3,430,455 3/1969 Stuart et al. ..62/383 envirgnments, 3,229,755 l/l966 Komarow ..62/383 UX 3,478,819 11/1969 Reinke ..165/32 11 Claims, 7 Drawing Figures ill | lllllll PATENTEB FEB 2 01973 sum 10F 5 Thomas P. Hosmer John F Peterson INVENTORS BY /iw-g 4 Attorney PAIENIED #5920 1975 3, 7, 01

sum 20F 3 Fig. 3

Thomas R Hosmer John F Peterson INVENTORS Fig.4 Q F 7 Attorney CRYOGENIC THERMAL SWITCH This invention relates to thermal switches and more particularly to devices designed to controllably connect and disconnect one or more thermal loads to a source of refrigeration.

In the use of cryogenic refrigeration to cool such thermal loads as cryopumping panels, detecting devices such as ir detectors, and specimens to be examined in one manner or another, it is sometimes necessary to be able to effect rapid cooldown of such loads. Because cryogenic refrigeration devices require a relatively long cooldown time, it is desirable to have the cryogenic refrigeration continuously available and to use a flow path or thermal switching means which may be controllably connected to the load for rapid cooling and precise temperature control. Such thermal switching means should be able to control the flow of thermal energy between two bodies at different temperatures and/or to vary the temperature of the refrigeration load. When the switch is completely engaged it should be of a kind which maintains a minimum temperature difference between the refrigeration source and thermal load; and when it is completely disengaged to break the thermal link between the load and refrigeration source it should be designed so that no drain of thermal energy is impressed on the refrigeration source other than the minimal leak to its surroundings.

The thermal switch of this invention comprises a fluidtight cylindrical housing, the wall of which is alternately formed of relatively thick copper sections (or other high heat conductivity, high heat capacity material) and relatively thin stainless steel sections (or other low heat conductivity, low heat capacity material). A plunger of copper is moved within the fluidtight cylinder to serve as a thermal link between the thickwalled copper sections. Helium fills the void volume within the housing, and means are provided to move the plunger and to thermally connect one of the thick copper sections to a refrigeration source and the remaining copper section or sections to thermal loads.

It is therefore a primary object of this invention to provide an improved thermal switch capable of providing a controllable heat flow path between a refrigeration source and one or more thermal loads. It is another object to provide a switch of the character described which can be manipulated accurately to control and vary the temperature of one or more thermal loads. It is yet another object of this invention to provide an improved thermal switch which can be engaged to deliver refrigeration from a continuous source to a load to effect rapid cooling of the load. It is still another object to provide a thermal switch which is of a design which makes it easy to cross physical barriers separating two different physical states such as two chambers maintained at two different pressure levels. Other objects of the invention will in part be obvious and will in part be apparent hereinafter.

The invention accordingly comprises the features of construction, combinations of elements, and arrangements of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which FIG. 1 is a longitudinal cross section of one embodiment of the switch of this invention which is thermally linked to a cryogenic refrigerator;

FIG. 2 is a cross section of the switch of FIG. 1, taken along line 2-2 of FIG. 1, showing one means by which the switch may be thermally linked to the refrigerator;

FIG. 3 is a longitudinal cross section of a portion of another embodiment of the switch of this invention designed to be thermally linked to two separate and distinct thermal loads;

FIG. 4 is a cross section of the switch of FIG. 3, taken along line 4-4 of FIG. 3, showing another means by which the switch may be thermally linked to a refrigerator;

FIG. 5 is a longitudinal cross section of yet another embodiment of the switch of this invention which is thermally linked to a reservoir of cryogenic fluid serving as the source of refrigeration;

FIG. 6 is a cross section of the switch of FIG. 5, taken along line 6-6 of FIG. 5, showing a means by which the switch may be thermally linked to the fluid reservoir; and

FIG. 7 illustrates, partly in cross section, a switch of this invention having magnetic means to turn it on and off.

The thermal switch illustrated in FIGS. 1 and 2 is suitable for connecting a thermal load to a cryogenic refrigerator in a manner to be able to accurately vary and control the temperature of the load. Such a load may typically be an ir detector or a sample to be physically examined, e.g., to have its ir absorption determined. In the following description those parts of the apparatus which are formed of a metal which exhibits relatively high thermal conductivity and relatively high heat capacity at cryogenic temperatures will, for convenience, be referred to as copper. However, it is meant to include any metal which possesses these desired characteristics including, but not limited to, silver and aluminum. Likewise those parts of the apparatus which are formed of a metal which exhibits low thermal conductivity and low heat capacity while retaining sufl'icient structural strength will be referred to as-stainless steel; but it is meant to include any metal which possesses these desired characteristics including, but not limited to, beryllium copper, constantan, silicon bronze and alloys of iron and nickel.

The thermal switch of FIG. 1 is formed of a cylindrical housing member 10 fluid-tightly sealed on the refrigeration delivery ends by means of a copper cap 1 l which in itself may form a heat station to which the load to be refrigerated is thermally bonded. The term heat station is used hereinafter to designate a high heat conductivity mass which connects a refrigeration source such as an expanding fluid or evaporating cryogen to some object to be refrigerated or to an auxiliary refrigeration transfer means such as a heat transfer path to a Joule-Thomson loop or a thermal link to some refrigeration load.

The cylindrical housing 10 is fluid-tightly sealed within the linear moving mechanism generally indicated at 14 and described in detail below. The cylindrical housing is formed of a thin-walled stainless steel section 15 which extends into linear moving mechanism 14, a first thick-walled copper section 16 which is thermally linked to the refrigeration source,

another thin-walled stainless steel section 17 and a second thick,walled copper section 18 which is sealed by copper capl l. A solid copper plunger 19, mounted on a rod 20 formed of a low conductivity material such as stainless steel, micarta or polytetrafluoroethylene, is positioned for movement within chamber 21 defined within the fluid-tight cylindrical housing. The void volume of chamber 21 is filled with helium, preferably above one atmosphere, e.g., up to about atmospheres. It is preferable that the annular spacing 22 between the plunger 29 and inner wall of the housing be kept at a minimum, e.g., about 0.0002 inch, at least for the thick-walled copper sections. In such an arrangement, thick-walled copper section 16 and thickwalled copper section 18 along with cap 11 are heat stations.

The refrigeration source, partially shown in FIG. 1, is a staged cryogenic refrigerator 23 having a heat station 24 associated with its coldest end. Such a refrigerator is described in U.S. Pat. No. 2,966,035 and a preferred type of heat station is described in a copending application Ser. No. 807,606 filed Mar. 17, l969, in the names of Fred F. Chellis and James A. O'and assigned to the ing the particular orientation illustrated in the drawings. The switch of this invention may be oriented in any desired manner.) Block 33 is affixed to a threaded collar 35 to form a pressure-tight seal, and collar 35 is in turn welded to the bottom end of the stainless steel section 15. Collar 35 is adapted to have a sealing means such as O-ring 36 associated with it to make a seal with an apertured mounting member 37 such as a barrier which can isolate the volume in which the switch itself is located from the volume in which the linear moving mechanism 14 may be located. The lower block 34 is fastened to a rod 38 which has a keyway 39 engageable by a pin 40 mounted in a ring 41 which in turn is affixed to the main barrel 42 of the linear moving mechanism. This main barrel 42 may contain calibrations such as external line markings 43 for position read-out in K, similar to a micrometer. Driving rod 31 is advanced or withdrawn by turning operating knob 44 which is threaded along its internal wall to engage threads in the bottom section 45 of the main barrel 42. A pivot block 46, located within the same assignee as the present application. It is, of

course, within the scope of this invention to use other types of cryogenic apparatus such as that described, for example, in U.S. Pat. Nos. 2,906,101, 2,657,553, and 1,275,507. It is also within the scope of this invention to use a circulating fluid cryogen as the source of refrigeration as illustrated in FIGS. 5 and 6. A copper disk 25 is thermally bonded to the end of heat station 24 which is cooled by the fluid within a refrigeration chamber surrounded by the heat station. A copper plate 26 is thermally bonded to the first thick-walled copper section 16 of the cylindrical housing and the copper disk 25 and plate 26 are thermally bonded to each other through an indium shim 27 under the force of screw 28. This provides the final thermal link between the refrigerator and the thermal switch.

Although the housing 10, plunger 19 and rod 20 are referred to as cylindrical and shown to be circular in cross section in the drawings, the term cylindrical is to be understood as being used in its broadest sense to encompass housings and rods of any suitable cross section including, but not limited to, oval, triangular, square and polygonal.

The thermal switch of FIG. 1 is designed to attain variable and accurately controlled temperature of the load associated with cap 11 serving in the role of a heat station. This, in turn, requires a accurately controllable linear moving mechanism, that shown in FIG. 1 representing one example of such a mechanism. It is necessary to move rod 20 up and down and to be able to determine the position of plunger 19 and relate its position to a predetermined load temperature for any one switch and refrigeration source. Rod 20 (typically of Micarta) is attached through an aligning member 30 to a stainless steel driving rod 31 which is encased partially in housing section 15 and partially in a bellows 32. The bellows 32 are fluid-tightly sealed to an upper cylindrical block 33, through which driving rod 31 moves, and to a lower cylindrical block 34. (It will be appreciated that upper and lower as used in describing the components of this switch are merely relative and are employed for convenience in describknob 44 against ring 47 and affixed to rod 38, permits rotation of knob 44 while the pin and keyway system prevents rotation or twisting of bellows 32. An end member 48 provides a cover for the operating knob 44 and serves to retain pivot block 46 by clamping it against ring 47. The fluid-tight domain which is filled with helium ends with the chamber 49 defined within the bellows.

In the operation of the thermal switch of FIGS. 1 and 2, the copper plunger 19 is moved within the housing to control the amount of refrigeration delivered to, and hence the temperature of,a thermal load associated with cap 11. If no refrigeration is to be delivered to such a load, then copper plunger 19 is withdrawn, by turning knob 44, preferably to a position such that the top of plunger 19 is essentially on the same level as the top of the thick-walled copper section'16 connected to the refrigerator. During those times when no refrigeration is delivered to a load the top of the plunger 19 may occupy a position anywhere between this preferred position and just below the lower level of the next thick-walled copper section 18. However, as the copper plunger approaches the lower level of the thickwalled copper section 18 some cooling of this wall section will be effected. The purpose of the heliumgas is to provide conduction across the narrow annular gap 22 between plunger 19 and the copper heat stations 16 and 18. Thus when the switch is disengaged, the distance between plunger 19 and copper wall section 18 is wide, conduction is poor and essentially no refrigeration is delivered to cap 11 and to a thermal load associated with it. When refrigeration of the cap and its associated thermal load is desired the knob 44 is turned to advance copper plunger 19 into the thickwalled section 18. When plunger 19 enters the thickwalled section 18 to define the narrow gap 22 between the outer surface of plunger 19 and the inner wall of section 18, the length of gap 22 determines the temperature of cap 11 and its associated load. The further the plunger is moved upwardly to lenghten this gap, the lower is the temperature of cap 11. Minimum temperature is reached when plunger 19 contacts cap 11. The position of knob 44 relative to the calibrated lines 43 on barrel 42 may therefore be calibrated for any one refrigeration source-switch combination to read directly in K of stabilized load temperature. As an example of the performance of the thermal switch of FIGS. land 2, if the fluid in the refrigeration chamber in refrigerator 23 which is used to cool heat station 24 is at 20K, then it is possible to cool a one-watt load thermally linked to cap 11 to 22K. Depending upon the load and the refrigeration source it is possible to achieve cooling over a range from about 15K to 200K.

The thermal switch of this invention is readily adaptable for use within two different physical domains since it can be installed to intersect a barrier between such domains in addition to barrier 37 which separates the switch from the linear drive mechanism 14. The switch can be mounted so that the barrier is intersected by the thin-walled stainless steel section 17 which can be further thermally insulated from the barrier if desired. As an example, a portion of the switch along with the refrigeration. source may be located within an evacuated chamber while the remaining portion of the switch and its associated thermal load may be located in a controlled atmosphere, the pressure of which may be greater or less than that of the chamber in which the refrigerator is located.

FIGS. 3 and 4, in which like reference numerals are used to refer to identical components of FIGS. 1 and 2, show a thermal switch adapted to refrigerate two separate loads and a modification of the means by which a heat station of a refrigerator may be thermally linked to the switch. The housing 10 of the thermal switch of FIG. 3 is constructed to have an additional thin-walled stainless steel section 51 and an additional thickwalled copper section 52 between copper section 16 to which the refrigerator is linked and copper section 18 which is linked to the load. The thick-walled copper section 52 may be thermally linked to a load, for example the thin-walled, high-surface area ring 53. Exemplary of the use of the thermal switch'of FIG. 3 is the cooling of an ir detector. The thin, high-surface ring 53 may serve as a cryopumping surface to condense and adsorb moisture from the environment in which the detector is located prior to the cooling of the detector itself which would be mounted for cooling on cap 11. The copper plunger is, in such an instance, moved first so that its top is at essentially the same level as the top of the thick-walled section 52. By maintaining the clearance passage 54 between the outer wall of the plunger 19 and the inner wall of thick-walled section 52 at a minimum, e.g., about 0.0002 inch, heat transfer may be effected across the helium within this clearance passage. Therefore, as plunger 19 is moved upwardly to enter that portion of chamber 21 defined by wall section 52, refrigeration is delivered to wall section '52 and hence to load ring 53. As the plunger is moved upwardly, the ring 53 becomes colder, finally reaching a temperature only a few degrees higher than heat station 24. Continuing the example of cooling an ir detector, after the ring 53 has been cooled for a time sufficient to condense and adsorb the moisture in the atmosphere surrounding the detector, the plunger is moved upwardly again into the thick-walled section 18 and into contact with the inner surface of cap 11, if desired, to obtain a predetermined temperature of the detector which is cooled and remains free of any condensed moisture.

The thermal switch of FIGS. 3 and 4 is linked to the heat station 24 of the refrigerator by means of a flexible copper strap 55 which is fastened to copper disk 25 by means of a screw 56 or the like and is curved around the outer surface of thick-walled section 16 to make thermal contact with it. It will be appreciated that the thermal linking systems shown in FIGS. 2 and 4 are readily interchangeable.

In the modification of the thermal switch of this invention shown in FIGS. 5 and 6, a liquid cryogen is used as the source of refrigeration, a different driving system is used to move the copper plunger and the housing is modified. The basic concept of the switch and its operation are however the same as that for the switches of FIGS. 1-4. In the thermal switch of FIGS. 5 and 6, the cylindrical housing 65 is formed of a plurality of sections, designated from right to left as a rack housing 66, a T-joint section 67 providing communication with another cylinder to be described, a thinwalled stainless steel section 68, a thick-walled copper section 69, a second thin-walled stainless steel section 70, and a second thick-walled copper section 71 ten minating in a solid copper plug 72. The copper plunger 19 is attached to a rack 73, preferably formed of a material exhibiting low thermal conductivity and having a low heat capacity. The width of the narrow annular spacing 74 between the plunger and the internal wall of the housing remains constant throughout in this embodiment.

A second fluid-tight cylinder 75 is connected with cylinder 65 through a fluid-tight T-joint 76. This cylinder 75 is formed of a tubing 77 which in turn is connected to a heavy tubing member 78. A pinion rod 79 connects pinion 80 with a hand-actuated knob 81 and it is encased in a rod casing 82 which, through sealing means not shown, makes a fluid-tight seal with heavy tubing member 78. Rod casing 82 and the inner wall of cylinder 75 define an annular spacing 83 which is connected through a valve-controlled conduit 84 to a source of high-pressure helium (not shown) and to the annular spacing 74 within cylinder 65.

In the embodiment illustrated in FIGS. 5 and 6, a cryogenic fluid, e.g., liquid helium, furnishes the required refrigeration. A vessel is provided to contain liquid helium 91, or other cryogen; and a copper mass 92 designed to fit around at least a portion of copper section 69 (see FIG. 6) defines at least a part of the bottom of vessel 90 to serve as a heat station. A valve-controlled insulated conduit 93 provides fluid communication between a source of liquid helium (not shown) and the interior of vessel 90. A conduit 94 having a one-way, pressure-controlled valve 95 provides for the discharge of vaporized helium from vessel 90. Around the liquid helium vessel 90 is an annularlyshaped vessel 96 for containing a liquid cryogen 97 such as nitrogen to serve as a radiation shielding for the helium vessel. The liquid nitrogen is introduced into vessel 96 through a valve-controlled conduit 98 and vaporized nitrogen gas is removed through conduit 99 having a one-way, pressure-controlled valve 100. In accordance with accepted practice in handling cryogenic refrigeration sources, an effective insulation means such as an evacuated chamber (not shown) will be used to surround the refrigeration source whether it is a refrigerator or a supply of a liquid cryogen.

In some uses of the thermal switch, it may not be necessary to be able to vary the temperature of the load over a range, but just operate it in on and off modes. For such cooling applications the switch may have a simple means for moving the copper plunger between two limits of travel. Such a simple means, magnetically operated, is shown somewhat schemati cally in FIG. 7 wherein like reference numerals are used to refer to like components of FIG. 1. in FIG. 7, the normal stainless steel housing (e.g., 15 of FIG. 1) is replaced by a nonmagnetic sheath tube 110 sealed at the bottom by cover 111. Around the sheath tube 110 are positioned two sets of coils 112 and 113 spaced apart a distance essentially corresponding to the distance between the two thick-walled copper sections 16 and 18. Each set of coils is contained within proper sheathings 114 and 115, respectively, and is connected to a suitable power source 116, through connection leads 117 and 118 for coils 112 and connection leads 119 and 120 for coils 113. A three-way switch 121 makes it possible to alternately connect coils 112 or 113 to the power source to cause the rod 20, which has a slug of magnetic material 122 affixed to its end, to move to its lowermost position to turn the thermal switch off or to move to an uppermost position within the magnetic field generated by coils 113 to turn it on.

The thermal switch of this invention is particularly suited for use in cryopumping devices, for cooling samples for physical analysis, and for cooling ir detectors and the like. It can also be incorporated in very small delicate instruments such as those used in cryosurgery.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim:

1. A thermal switch adapted to effect a thermal linkage between a source of refrigeration and a thermal load, comprising in combination a. a fluid-tight cylindrical housing, the wall of which is alternately formed of relatively thick-walled sections of a first material having relatively high thermal conductivity and high heat capacity at cryogenic temperatures and of relatively thinwalled sections of a second material having relatively low thermal conductivity and low heat capacity at cryogenic temperatures, said housing terminating with one of said thick-walled sections sealed with an end member of said first material;

b. a plunger free from any permanent connection with said housing and movable within said housing, said plunger being formed of a material having relatively high thermal conductivity and high heat capacity at cryogenic temperatures, and defining with the wall of said housing a narrow annular spacing;

c. helium filling the void volume within said housing;

d. means to move said plunger connected to said plunger and including means external of said housing which are adapted to effect the movement of said plunger within said housing;

e. thermal connection means adapted to thermally link a source of refrigeration with that thick-walled section most remote from said thick-walled section sealed with an end member.

5 2. A thermal switch in accordance with claim 1 wherein said first material is copper and said second material is stainless steel.

l. bellows means affixed to said housing means and forming a part thereof, 2. driving rod means attached to said plunger and in axial alignment therewith located within said hous- 2 ing means, and

3. means external of said housing for effecting linear motion of said driving rod means and of compressing and extending said bellows means.

6. A thermal switch in accordance with claim 5 wherein said means for effecting linear motion of said driving rod means comprises, in combination 1. a calibrated barrel surrounding said bellows means,

2. a rotatable operating knob adapted for linear motion along said barrel, and

3. mechanical linking means connecting said knob with said driving rod and said bellows and including means to prevent twisting of said bellows as said operating knob is rotated.

7. A thermal switch in accordance with claim 1 wherein said means to move said plunger comprises, in combination 1. rack means affixed to one end of said plunger,

2. pinion means engageable with said rack means,

and

3. means to rotate said pinion.

8. A thermal switch in accordance with claim 1 wherein there are two thick-walled sections of said housing and said means to move said plunger comprises, in combination 1. a nonmagnetic sheath tube forming an extension of said housing,

2. first and second coil means adapted to generate first and second magnetic fields, said coils means surrounding said sheath tube and being separated by a spacing essentially equal to the thin-walled section of said housing separating said thickwalled sections,

. a driving rod attached to and in axial alignment with said plunger,

. a magnetic mass attached to said driving rod, and

. means to selectively energize either said first or second coil means to position said magnetic mass in said first or second magnetic field and thereby to position said plunger within one or the other of said thick-walled sections.

LII-b 9. A thermal switch in accordance with claim 1 wherein said thermal connection means comprises a copper plate thermally bonded to said thick-walled section and means adapted to form a thermal bond between said plate and the heat station of a source of refrigeration.

10. A thermal switch in accordance with claim 1 wherein said thermal connection means comprises a copper strap bent to be thermally bonded around a portion of the outer wall of said thickwalled section and 

1. a nonmagnetic sheath tube forming an extension of said housing,
 1. rack means affixed to one end of said plunger,
 1. a calibrated barrel surrounding said bellows means,
 1. bellows means affixed to said housing means and forming a part thereof,
 1. A thermal switch adapted to effect a thermal linkage between a source of refrigeration and a thermal load, comprising In combination a. a fluid-tight cylindrical housing, the wall of which is alternately formed of relatively thick-walled sections of a first material having relatively high thermal conductivity and high heat capacity at cryogenic temperatures and of relatively thin-walled sections of a second material having relatively low thermal conductivity and low heat capacity at cryogenic temperatures, said housing terminating with one of said thick-walled sections sealed with an end member of said first material; b. a plunger free from any permanent connection with said housing and movable within said housing, said plunger being formed of a material having relatively high thermal conductivity and high heat capacity at cryogenic temperatures, and defining with the wall of said housing a narrow annular spacing; c. helium filling the void volume within said housing; d. means to move said plunger connected to said plunger and including means external of said housing which are adapted to effect the movement of said plunger within said housing; e. thermal connection means adapted to thermally link a source of refrigeration with that thick-walled section most remote from said thick-walled section sealed with an end member.
 2. A thermal switch in accordance with claim 1 wherein said first material is copper and said second material is stainless steel.
 2. driving rod means attached to said plunger and in axial alignment therewith located within said housing means, and
 2. a rotatable operating knob adapted for linear motion along said barrel, and
 2. pinion means engageable with said rack means, and
 2. first and second coil means adapted to generate first and second magnetic fields, said coils means surrounding said sheath tube and being separated by a spacing essentially equal to the thin-walled section of said housing separating said thick-walled sections,
 3. means to rotate said pinion.
 3. a driving rod attached to and in axial alignment with said plunger,
 3. mechanical linking means connecting said knob with said driving rod and said bellows and including means to prevent twisting of said bellows as said operating knob is rotated.
 3. means external of said housing for effecting linear motion of said driving rod means and of compressing and extending said bellows means.
 3. A thermal switch in accordance with claim 1 wherein said plunger is formed of copper.
 4. A thermal switch in accordance with claim 1 wherein there are at least three of said thick-walled sections, the sections between said thick-walled section sealed with an end member and said thick-walled section most remote therefrom having means adapted to thermally link them to refrigeration loads.
 4. a magnetic mass attached to said driving rod, and
 5. means to selectively energize either said first or second coil means to position said magnetic mass in said first or second magnetic field and thereby to position said plunger within one or the other of said thick-walled sections.
 5. A thermal switch in accordance with claim 1 wherein said means to move said plunger comprises, in combination
 6. A thermal switch in accordance with claim 5 wherein said means for effecting linear motion of said driving rod means comprises, in combination
 7. A thermal switch in accordance with claim 1 wherein said means to move said plunger comprises, in combination
 8. A thermal switch in accordance with claim 1 wherein there are two thick-walled sections of said housing and said means to move said plunger comprises, in combination
 9. A thermal switch in accordance with claim 1 wherein said thermal connection means compriseS a copper plate thermally bonded to said thick-walled section and means adapted to form a thermal bond between said plate and the heat station of a source of refrigeration.
 10. A thermal switch in accordance with claim 1 wherein said thermal connection means comprises a copper strap bent to be thermally bonded around a portion of the outer wall of said thickwalled section and adapted for attachment to the heat station of a refrigeration source. 